Field devices, such as process variable transmitters, are used in the process control industry to remotely sense process variables. One type of field device or process variable transmitter includes a capacitive pressure sensor for monitoring a process pressure.
Various dielectric materials have been used in these capacitive pressure sensors. For example, fluid filled constructions are sometimes employed in which an oil or other fluid is used as the dielectric material. In other constructions, it is desired to use a vacuum as the dielectric material. Vacuum dielectrics can be of lower complexity constructions, can reduce costs, are non-oil filled, can be more accurate, and/or can be more stable. However, because of the vacuum dielectric construction, these sensors typically only measure absolute pressure, and not the gage pressure of the process (e.g., the process pressure relative to a local pressure, such as the surrounding atmospheric or barometric pressure). Gage pressure is also sometimes known as “gauge” pressure.
Prior pressure sensor configurations for measuring the gage pressure of a process have utilized a number of different approaches. For example, high pressure sensors have been sold as sealed packages, with a calibrated pressure offset built-in to approximate gage pressure. Other constructions have added a second sensor to measure the barometric pressure, and have adjusted the measured pressure using electronics or software to provide a gage pressure estimate. This adds cost to the pressure sensor. Still other constructions have allowed atmosphere (air) to become the dielectric material, which can result in temperature instability errors and can limit the sensors ability to be used in corrosive environments. With these various attempts commonly having detrimental effects on accuracy, cost, stability or reliability, new pressure sensor configurations which can accurately measure gage pressure of a process without suffering these detrimental effects would be very beneficial.